For the relief of postoperational pain and in the case of cancer patients in an advanced stage of the disease morphine and its derivatives are the most frequently used efficient analgesics. It is a well known fact, however, that in patients treated with morphine, harmful side-effects, e.g., respiratory depression, tolerance, and dependence, develop in a relatively short time. The patient gets used to morphine and rapidly rising doses are required to obtain an equianalgesic effect; tolerance or dependence develops and the patient is in permanent need of the euphorizing effect of morphine. Another disadvantage of morphine is the fact that it is practically ineffective on oral application.
All the analgesics suited for the treatment of unbearable pain (e.g. cancer, postoperative conditions, infarction, lithiases, etc.) are liable to induce the development of tolerance on chronic administration and their withdrawal produces severe-- often fatal-- somatic and psychic symptoms (physical and psychic dependence). It is generally accepted (Martin, 1967, Pharm. Rev. 19, 463) that the appearance of tolerance and dependence necessarily accompany the action of the morphine type drugs on the analgesic receptors. An analgesic equipotent to morphine, but devoid of its narcotic side-effects, has for long been needed in clinical practice.